1. Field of the Invention
The present invention relates to methods and apparatus for the separation of a catalyst and hydrocarbon materials in a fluidized catalytic cracking (FCC) unit and for subjecting the FCC catalyst to multistage stripping. More particularly, the present invention relates to an improved method and apparatus for reducing the contact time between catalyst and hydrocarbon materials within an FCC riser and after exiting from an FCC riser prior to separation by novel or conventional separating means.
2. Discussion of the Prior Art
The field of catalytic cracking and particularly fluid catalyst operations have undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalyst and particularly crystalline zeolite cracking catalyst, new areas of operating technology have been encountered requiring even further refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition with a hydrocarbon feed generally upwardly through one or more riser conversion zones (FCC cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst.
Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. During the hydrocarbon step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion zone. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by a separator, such as cyclonic equipment, and/or stripping gas in a separate catalyst stripping zone. Hydrocarbon conversion products separated and stripped from the catalyst are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
Of particular interest has been the development of methods and systems for separating catalyst particles from a gasiform mixture exiting the riser and containing catalyst particles and vaporous hydrocarbon product materials, particularly the separation of high activity crystalline zeolite cracking catalyst particles, under more efficient separating conditions so as to reduce overcracking of hydrocarbon conversion products and promote the recovery of desired products of a hydrocarbon conversion operation. Typically the gasiform mixture exiting the riser and containing catalyst particles and vaporous hydrocarbon product materials will exit the riser through a perpendicular conduit which is attached to cyclonic equipment. The cyclonic equipment is now typically used for efficient separation of fluidizable catalyst particles from the gasiform mixture. Passing gas from a riser to a perpendicular conduit can result in substantial erosion of the top of the riser where catalyst impinges before passing into the conduit. The impinging also has the disadvantage of increasing the residence time of catalyst within the riser. Another drawback is that the cyclonic equipment typically used for efficient separation of fluidizable catalyst particles from the gasiform mixture exiting from the riser, often permits an undesirable residence time of the product vapor within a cyclone. This extended residence time both from the impinging and within the cyclone reduces the desired product yield by as much as 4% through non-selective cracking. Recent developments in this art have been concerned with the rapid separation and recovery of entrained catalyst particles from the gasiform mixture.
Various processes and mechanical means have been employed heretofore to effect rapid separation of the catalyst phase from the hydrocarbon phase, at the termination of the riser conversion zone, to minimize contact time of the catalyst with cracked hydrocarbons.
U.S. Pat. No. 4,043,899 to Anderson et al discloses a system in which a mixture of catalyst and hydrocarbon gases pass from a riser into a stripping cyclone. Steam is admitted into the stripping cyclone to strip catalyst while in the cyclone.
U.S. Pat. No. 4,219,407 to Haddad et al describes a riser cracking-catalyst regeneration operation wherein a gasiform mixture discharged from a riser is passed into conduits with downwardly directed end portions to discharge a concentrated stream of catalyst separated from gasiform material into a downcomer zone and out of further contact with discharged gasiform material in the gasiform mixture. A stripping gas is used to particularly aid the separation of catalyst from the hydrocarbon vapors.
U.S. Pat. No. 4,206,174 to Heffley et al discloses an apparatus provided contiguous with a riser discharge which centrifugally initially separates a solid and gasiform mixture into a solid phase and a gasiform material phase prior to discharge from a riser into a cylindrical vessel. The centrifugal separation is provided by passing the gasiform mixture across stator type blades (fixed spaced apart sloping baffles), at or near the downstream end of the riser, to impart a spinning motion to the gasiform mixture. The catalyst may be further stripped, if desired, in equipment above and about the riser discharge.
While the above patents represent improvements in the field of rapid separation and stripping of hydrocarbon materials from catalyst particles, there is still a need to further reduce total contact time between hydrocarbon materials and catalyst to reduce to the extent possible non-selective cracking.
For example, a problem associated with the system of U.S. Pat. No. 4,206,174 to Heffley et al is that catalyst exiting the riser is separated from gasiform material only by centrifugal force. In addition, it is difficult to achieve significant catalyst stripping within the cylindrical vessel, thus extending total contact time. A difficulty with the system of U.S. Pat. No. 4,219,407 to Haddad et al and U.S. Pat. No. 4,043,899 to Anderson et al is that upwardly directed catalyst particles "impinge" off the top of a riser back into the continuous upward flow of hydrocarbon and catalyst. Thus, the downward velocity of the impinged particles must be reduced by the rising hydrocarbon/catalyst gasiform mixture before again moving upwardly in the riser toward its exit into a separating means. The additional contact time of hydrocarbon vapor and catalyst caused by the impinging effect produces overcracking and loss of precise control of cracked products.